Heat treatment of refractory metals



United States Patent 3,194,697 HEAT TREATMENT OF REFRACTORY METALSWinston H. Chang, Cincinnati, Ohio, assignor to General ElectricCompany, a corporation of New York No Drawing. Filed Sept. 28, 1962,tier. No. 227,045 7 Claims. (Cl. 148-133) This invention relates to amethod for heat treating refractory metal based alloys and, moreparticularly, to a heat treatment for refractory metal based alloyshaving a carbon content greater than about 0.05 weight percent tocontrol the amount, size and form of carbon present in the alloy.

The primary function of carbon in many refractory metal alloys is toform a metal carbide strengthener.

-" 'llhe precipitation of fine carbide particles lends high temperaturetensile and rupture strength to many refractory metals such as thosebased on molybdenum, tungsten, columbium, tantalum and chromium.However, when more than about 0.05 weight percent carbon andparticularly when more than about 0.1 percent carbon is included in suchalloys to increase strength, initial processing requires procedureswhich raise the temperature of the alloy sufficiently high to placeenough carbon in solution so that it later can be precipitated as finecarbides through additional processing. One of the dangers ofintentionally including relatively large amounts of carbon in solutionin such alloys is that relatively large quantities of carbon may remainin solution after processing because such processing cannot bring aboutfull carbide precipitation. Large quantities of carbon in solution actto the detriment of low temperature ductility.

It is an object of the present invention to provide a method for heattreating refractory metal alloys including relatively large amounts ofcarbon so that the use and distribution of the carbon is optimized andthe detrimental effects Which can result from too much carbon remainingin solution after subsequent processing is reduced.

- Another object of this invention is to provide a method ofprecipitating as a carbide strengthener, prior to the alloys completeprocessing, that portion of the carbon in solution in a refractory metalalloy which might not otherwise have been precipitated and which insolution would be detrimental to lower temperature ductility.

.These and other objects and advantages will be more readily understoodfrom the following description and examples which are typical of, butnot meant to be limitations on this present invention.

Briefly, the present invention in one form provides, in a method forheat treating a refractory metal based alloy, including carbon in excessof about 0.05 weight percent, the steps of aging the alloy prior tofinal processing, for a time and at a temperature suflicient toprecipitate fine metal carbides, and then reducing the alloy to afinished ,7 form.

Excessive carbon retained in solution for the life of a finished articlehas been found to result in low room temperature ductility. According toknown processing procedures, as a relatively high carbon-bearingrefractory metal based alloy is first heated at high temperatures,

m various portions of the carbon are dissolved, the amount at'a somewhatlower temperature, small particles of carbides begin to precipitate. Atthis point the microstructure will have the appearance of a solutionmatrix in idfiififi? Patented July 13, 1965 which the larger particlesof carbon still are present but there now exist smaller particles ofcarbides which have been precipitated at the processing temperature tostrengthen the alloy. Of the carbon that originally was in solution,part has precipitated and part has remained in solution. The part whichhas remained in solution is that which has been found to be detrimentalto low temperature ductility and that which the present inventionattempts to control and use.

This invention recognizes that by introducing an aging step at aparticular point in the-processing to precipitate carbides additional tothose which normally would be precipitated during subsequent working,the carbon which would normally remain in solution to the detriment oflow temperature ductility not only is removed but also is usedbeneficially. Thus the alloy is strengthened while at the same time lowtemperature ductility is improved.

The particular point referred to in the processing for the introductionof the aging step is a point prior to final processing such as rolling,swaging and the like. For example, if an article is to be finallyprocessed or finished directly from a casting, then the aging step wouldbe applied to the casting. If the article is originally cast, thenextruded, forged and finally swaged, the aging step would be introducedprior to swaging. If the final processing of an alloy would be rollinginto sheet, the aging step would be introduced prior to the rollingprocess. Thus the present invention controls the carbon content ofrefractory metal alloys to the greatest advantage.

In a preferred form, the aging step of this invention when applied toMo-base or W-base alloys is conducted for at least about 5 hours andless than about hours, to avoid overaging to the detriment of hightemperature strength, in a temperature range of about 2400-2800 F. Whenapplied to Cb-based or Ta-based alloys, the preferred aging step is atabout 200-2500 F. for about 1-10 hours; and when applied to Cr-basedalloys, the preferred aging is at about l8002300 F. for about l-10hours.

The annealing temperature for any of these alloys to solution carbon ispreferred within the following ranges to control the carbon placed insolution: for Mo based and W based alloys-4 200 to 4000" F.; forTa-based alloys-6000 to 4000 F.; for Cb-based alloys-3000 to 3500 F.;and for Cr-based alloys2500 to 3000 F. Annealing temperatures in excessof the higher temperature listed will tend to introduce more carbon intosolution than can be precipitated effectively in subsequent processing.I

The following Table 1 gives the composition of some Mo-based alloyswhich have been studied in connection with the present invention.

Table 1 Com osition wt. ercent Alloy p p Ti Zr 0 M0 1 0. 1 0. 14 Bal. 1.8 0. 13 Bal. 1. 6 0. 1 0. 13 Bal. 1. 6 0. 6 0. 13 Bal. 1 0. 1 0. ()1Ba].

The following Table 2 presents processing conditions and resultingstrength data for the alloys of Table 1 with conditions A-4, B-2, C-2and D-2 including the particular aging process of the present inventionprior to final working. Alloy V is included in Table 2 to show theeffect of comparable heat treatment on a relatively low carbon bearingMo-based alloy.

Table 2 Tensile properties 100-hr. Test rupture Alloy Processingconditions temp, strength,

F. U. E1. (1"), k.s.i.

k.s 1 percent I A. Extruded at 3,200 F., forged at 3,200-

2,800 E: A-l: Annealed at 3,000 F. 78 121.0 12

Swaged at; 2,5502,200 F., stress re- 2, 200 57.0 16 32.0 lieved (S.R.)at 2,200 I i/1 hr. A-2: Annealed at 3,500 I 78 132,

Swaged and S.R. as in A-l. 2, 200 63. 5 16 48. 0 A-3: Annealed at 3,750F. 78 106. 0 0

Swagcd and S.R. as in A-l. 2,200 78.0 16 50.0 A-4: Annealed at 3,750 F.and aged at 78 125 2 2,750 F./l6 hrs. Swaged and SR. as in A-l. 2,20073. 6 18 48. 0 II B. Extruded at 3,000 F;

B-l: Swaged 93% at 2,5002,100 F. 78 121. 8 1

S.R. 2,100 F. 1 hr. 3, 000 21. 5 37 13-2: Aged 2,500 F./50 hrs. 78 133.024

Swaged and S.R. as in 13-1. 3, 000 22. 7 31 III C. Extruded at 3,500 F.:

(1-1: Swaged 93% at 2,5002,l00 F. 78 139. 8 2

S.R. 2,100 F./1 hr. 3, 000 27.1 29 C2: Aged at 2,500 F./50 hrs. 78 133.2 18 N Swaged and SR. as in C-1. 3, 000 28. 3 36' IV D. Extruded at3,500 F.:

D-l: Swaged 93% at 2,5002,100 F. 78 162.3 13

8.3. 2,l00 F.1l1r. 3,000 31.3 '29 D-Z: Aged at 2,500 F./50 hrs. 78 148.0 23 Swagcd and SB. as in D-l. 3, 000 30. 7 V E. Extruded at 2,900 I";

E-l: Annealed at 2,600" F.

Swaged 88% at 2,200 F.1,800 F. 78 98. 0 33 S.R. at 1,800 F./1 hr. 2,20046.8 16 22. 0 12-2: Annealed at 3,500" F. 78 112. 0 36 Swaged and S.R.as in E-l. 2, 200 06.8 12 32. 0,

As used in Table 2 the terms U.T.S. means Ultimate Tensile Strength,k.s.i. means thousands of pounds per square inch, El means Elongationand S.R. means stress relieved. In Table 2, the conventional processingconditions for Alloy I is represented by A1.

Although the high temperature tensile and rupture properties of arelatively high carbon alloy such as Alloy I of Table 2 can be improvedby high temperature annealing, it is to be noted that this gain is atthe expense of room temperature ductility. For example the highertemperature processing of A-2 and A-3 results in more carbon being takeninto solution for subsequent precipitation and hence strengthening ofthe alloy. However, more carbon remains in solution as well and itsdetrimental effect is shown by the ductility data; A comparison of theroom temperature elongation for conditions A-2 and A-3 in view of theprogressively increasing annealing temperatures shows that as theannealing tempreature is increased and more carbon is taken .intosolution without being subsequently precipitated, the more brittle isthe alloy at lower temperature. More carbon is in solution after swagingand stress relieving to affect the low temperature ductility, which incase of condition A-3 is about 0%. However, comparing the data for condition A-3 with that for A-4 it is seen that the introduc-- tion of anaging step according to this invention to eliminate solutioned carbon byprecipitating additional carbides, prior tofinal swaging and stressrelief results in an alloy of improved low temperature strength andductility with comparable high temperature strength. This Alloy-I heattreated according to the present invention, as represented by conditionA-4 of Table 2, would be useful at room temperature as well as at 2200F. Thus Alloy I,

processed according to A-4, presents a ditferentkind of alloy, withregard to microstructure, for subsequent working. That same alloy heattreated in a conventional manner, as represented by A-1, would not be asstrong. Heat treated at high temperatures to dissolve more carbon forstrengthening, as shown by A3, it would be too brittle at lowertemperatures. I

A comparison of the data for alloys II, III and IV of Table 2 with andwithout the aging step in the heat treatment of the present inventionshows the substantial increase in low temperature ductility possiblefrom use of this invention. These alloys are particularly designed foruse with the aging heat treatment of this invention.

Alloys II and III, specifically designed to be treated by the heattreatment of this invention, showin particular the significant effect ofthe aging step. Alloy IV is a unique. and unusual alloy in that it isstrong and ductile at room temperature and 3000 F. with or without theaging step of the present invention. However, data for Alloy IV isincludedin Table 2 to show that its ductility can be further increasedthrough the processing of this present invention. Alloys II and III aremore sensitive to the heat treatment of the present invention. As shownby the data for conditions B2 and C-2, they exhibit adequate lowtemperature strength alloy with goodductility after such heat treatment.At the same time their high temperature strength is improvedby the agingstep of'the present invention.

Thus the data of Table 2 shows the effect of appropriate aging toprecipitate excess dissolved carbon as carbides before final processingof relatively high carbon-bearing alloys so that it is not available toact in a manner detrimental to low temperature ductility. This wasaccomplished in the examples of Table. 2 by introducing the aging stepprior to the final processing of swaging;

As a result of the aging process of the present invention, a fineprecipitate of carbide is present in the structure of a processed alloy.Therefore a different kind of structure is available for such subsequentsecondary processing as swaging and stress relief than is availableafter conventional hea-t'treatrnents. The aging step results in a verylarge volume: fraction of precipitated carbides which, uponsubsequentswaging-or working, increases the effectiveness of suchworking. Therefore, the comparison after swaging of a relatively highcarbonbearing alloy aged according to this invention, and thushaving'more precipitated carbides and less carbon in solution, with onenot so processed shows the aged alloy to be significantly more ductilewhile of comparable strength. This is shown by the .data in Table 2.

The aging process of the present invention does not have the samebeneficial effect on leaner carbon alloys for example, those. havingless than about "0.05 Weight percent carbon. As shown by Alloy V inTable2, an

alloy which is the same as Alloy I, except for the carbon content, whenannealed or aged at 2600 F. (E1) is lower both in strength and ductilitythan the alloy when annealed at 3500 F. (E-2). With higher carboncontent alloys, the higher annealing temperature of E2 type processingwould dissolve more carbon, thus to increase rather than decreasestrength but at a sacrifice of lower temperature ductility. This is nottrue of lower carbon content Alloy V because the smaller amount ofcarbon in Alloy V does not create the problem of carbon in solution,which problem is solved by this invention.

Although the present invention has been described in connection withspecific alloys as examples, particularly molybdenum base alloys, andwith specific times and temperatures, it will be recognized by thoseskilled in the art of metallurgy and heat treatment the variations andmodifications of which the present invention is capable.

What is claimed is:

1. In a method for heat treating a metal alloy based on war refractorymetal selected from the group consisting of M0, W, Cb, Ta and Crincluding carbon in the range of about 0.1-0.2 weight percent, the stepsof: annealing the alloy at a temperature of about 2500-4000 F. for atime sutficient to place carbon in solution with the base metal; agingthe alloy prior to final processing for about 1-50 hours at atemperature of about 1800-2800 F. to precipitate fine metal carbides inthe microstructure of the alloy; and then reducing the alloy to afinished form.

2. In a method for heat treating a Mo based alloy including carbon inthe range of about 0.10.2 weight percent, the steps of: heating thealloy at a temperature of about 3200-4000 F. for a time sufficient toplace carbon in solution with the base metal; aging the alloy prior tofinal processing for about 5-50 hours at about 2400- 2800 F. toprecipitate fine metal carbides in the microstructure of the alloy; andthen reducing the alloy to a finished form.

3. In a method for heat treating a W based alloy including carbon in therange of about 0.1-0.2 weight percent, the steps of: heating the alloyat a temperature of about 3200-4000 F. for a time sufiicient to placecarbon in solution with the base metal; aging the alloy prior to finalprocessing for about 5-50 hours at about 2400- 2800 F. to precipitatefine metal carbides in the microstructure of the alloy; and thenreducing the alloy to a finished form.

4. In a method for heat treating a Cb based alloy including carbon inthe range of about (1.1-0.2 weight percent, the steps of: heating thealloy at a temperature of about 3000-3500 F. for a time sufiicient toplace carbon in solution with the base metal; aging the alloy prior tofinal processing for about 1-10 hours at about 2000-2500" F. toprecipitate fine metal carbides in the microstructure of the alloy; andthen reducing the alloy to a finished form.

5. In a method for heat treating a Ta based alloy including carbon inthe range of about 0.1-0.2 weight percent, the steps of: heating thealloy at a temperature of about 3000-4000 F. for a time sufiicient toplace carbon in solution with the base metal; aging the alloy prior tofinal processing for about 1-10 hours at about 2000- 2500 F. toprecipitate fine metal carbides in the microstructure of the alloy; andthen reducing the alloy to a finished form.

6. In a method for heat treating a Cr based alloy including carbon inthe range of about 0.1-0.2 weight percent, the steps of: heating thealloy at a temperature of about 25003000 F. for a time sutficient toplace carbon in solution with the base metal; aging the alloy prior tofinal processing for about 1-10 hours at about 1800-2300 F. toprecipitate fine metal carbides in the microstructure of the alloy; andthen reducing the alloy to a finished form.

'7. In a method for heat treating a metal alloy based on a refractorymetal selected from the group consisting of Mo, W, Cb, Ta and Cr,including carbon in the range of about 0.050.2 weight percent, the stepsof: heating the alloy at a temperature of about 2500-4000 F. for a timesufficient to place carbon in solution with the base metal;subsequently, prior to final processing, aging the alloy for about 1-50hours at a temperature of about 1800-2800" F. to precipitate fine metalcarbides in the rnicrostructure of the alloy; and then reducing thealloy to a finished form.

References Cited by the Examiner UNITED STATES PATENTS 2,467,675 4/49Kurtz et al -174 2,628,926 2/53 Ramage et al. 148-11.5 2,678,272 5/54Ham et al. 14811.5 2,698,892 1/55 Hardin 75176 4/58 Ruthardt 148'-11.5

1. IN A METHOD FOR HEATING TREATING A METAL ALLOY BASED ON A REFRACTORYMETAL SELECTED FROM THE GROUP CONSISTING OF MO, W, CB, TA AND CRINCLUDING CARBON IN THE RANGE OF ABOUT 0.1-0.2 WEIGHT PERCENT, THE STEPSOF: ANNEALING THE ALLOY AT A TEMPERATURE OF ABOUT 2500-4000*F. FOR ATIME SUFFICIENT TO PLACE CARBON IN SOLUTION WITH THE BASE METAL; AGINGTHE ALLOY PRIOR TO FINAL PROCESSING FOR ABOUT 1-50 HOURS AT ATEMPERATURE OF ABOUT 1800-2800*F. TO PRECIPITATE FINE METAL CARBIDES INTEH MICROSTRUCTURE OF THE ALLOY; AND THEN REDUCING THE ALLOY TO AFINISHED FORM.